Evidence for mechanical coupling and strong Indian lower crust beneath southern Tibet

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How surface deformation within mountain ranges relates to tectonic processes at depth is not well understood. The upper crust of the Tibetan Plateau is generally thought to be poorly coupled to the underthrusting Indian crust because of an intervening low-viscosity channel1. Here, however, we show that the contrast in tectonic regime between primarily strike-slip faulting in northern Tibet and dominantly normal faulting in southern Tibet requires mechanical coupling between the upper crust of southern Tibet and the underthrusting Indian crust. Such coupling is inconsistent with the presence of active ‘channel flow’ beneath southern Tibet, and suggests that the Indian crust retains its strength as it underthrusts the plateau. These results shed new light on the debates regarding the mechanical properties of the continental lithosphere2, 3, 4, and the deformation of Tibet1, 5, 6, 7, 8, 9, 10.

At a glance


  1. Tectonic regime within and around the Tibetan Plateau.
    Figure 1: Tectonic regime within and around the Tibetan Plateau.

    a, Principal axes of the horizontal components of the earthquake moment tensors, normalized to the length of the largest axis (red is compression, blue is extension). b, c and d, Focal mechanisms of upper crustal (depth less than 50km) earthquakes of moment magnitude exceeding 5.5, subdivided on the basis of rake. Black focal mechanisms are from the studies listed in the Supplementary Information; grey focal mechanisms are well-constrained CMT solutions (http://www.globalcmt.org/; over 50% double couple; ref. 30). d also shows the India–Asia convergence velocity23. The dashed line in the central plateau on each panel shows the estimated location of the northern limit of underthrust Indian lithosphere12, 19.

  2. Modelled principal axes of the horizontal strain-rate tensor at the surface.
    Figure 2: Modelled principal axes of the horizontal strain-rate tensor at the surface.

    Red bars represent compression, and blue bars extension. Red and blue crosses (with bars of equal length) indicate strike–slip deformation. North of the northernmost dashed line, the lower 35km of the crust is given the velocity of Tarim relative to India. For a and b, south of the southernmost dashed line the lower 20km of the crust is forced to have zero velocity. c is the same as a, except that between the two southernmost dashed lines a horizontal decoupling horizon is inserted above the rigid lower crust. Background shading represents elevation. See Supplementary Information for the modelled velocities. Scale bars are strain rate, 2×10−8yr−1.


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Author information


  1. Tectonics Observatory, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA

    • Alex Copley,
    • Jean-Philippe Avouac &
    • Brian P. Wernicke
  2. Present address: Bullard Labs, Department of Earth Sciences, University of Cambridge, CB3 0EZ Cambridge, UK.

    • Alex Copley


A.C. performed the calculations, A.C., J.-P.A. and B.W. discussed the results, and A.C. and J.-P.A. wrote the manuscript.

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The authors declare no competing financial interests.

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Supplementary information

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  1. Supplementary Information (2.4M)

    This file contains Supplementary Figures 1-3 with legends and Supplementary References for the earthquake focal mechanisms show in black in Figure 1 of the main paper.

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